For mobile phones, batteries with a higher capacity are desired, but it is quite difficult to increase the capacity of secondary batteries. Thus direct methanol fuel cells (DMFC) using methanol fuel become of greater interest.
DMFC has advantages such as possible size reduction since they can utilize liquid fuel directly without converting it into hydrogen or the like. Research efforts have been made thereon toward commercial use. However, the problems that the electrolyte membrane has a high methanol permeability and the anode catalyst has a low methanol oxidation activity arrest the commercial application of DMFC.
Most often PtRu base catalysts are used as the anode catalyst. Due to their low methanol oxidation activity, PtRu catalysts must be used in large amounts, typically of about 3 to 10 mg/cm2. As the amount of PtRu catalyst increases, the catalyst layer becomes substantially thicker. For this reason, PtRu black catalyst is typically used to facilitate diffusion of methanol as the fuel. However, the PtRu black catalyst has such a particle size of at least 5 nm that they have a low methanol oxidation activity per catalyst mass and a high fraction of PtRu not contributing to catalytic reaction. It is thus preferred to use a catalyst having a good distribution of smaller PtRu particles loaded on a conductive carbon support. It is also desirable to maximize the amount of PtRu particles supported on carbon in order to minimize the thickness of a catalyst layer.
Making extensive investigations to attain the above object, the inventor found that when a process of forming metal nuclei such as Pt on support carbon, then growing PtRu on the metal nuclei (to be referred to as two-stage loading process, hereinafter) is used as means for producing a highly loaded/highly dispersed PtRu-laden catalyst, there is obtainable a catalyst in which PtRu particles having an average particle size of up to 4 nm are loaded on carbon in a well dispersed fashion even at a loading of at least 50% by mass (Patent Document 1: JP-A 2007-134295). This two-stage loading process yields a catalyst having a methanol oxidation activity 2.5 times higher than the commercially available catalyst TEC61E54 (Tanaka Kikinzoku Group). However, a further improvement in methanol oxidation activity is desired so that the catalyst may find commercial application.
Means contemplated for improving methanol oxidation activity include further atomization of catalyst particles and enhancement of catalyst activity per unit surface area. For example, a study is made on the catalyst composition, for example, addition of a third metal such as Rh or Ir besides PtRu (Non-Patent Document 1: Kawaguchi et al., Catalyst 46 (6), 417-419, 2004). The catalysts under study have a loading as low as 30% by mass while a catalyst activity corresponding to a practical loading, for example, of at least 50% by mass is unknown.
There is a desire for a further improvement in methanol oxidation activity of a catalyst having a maximum loading independent of whether the improvement resorts to the extension of surface area or the increase in activity per unit surface area or both.                Patent Document 1: JP-A 2007-134295        Non-Patent Document 1: Kawaguchi et al., Catalyst 46 (6), 417-419, 2004        